To protect regional or global environment, harmful materials are removed from exhaust gases emitted from engines of automobiles, etc., using exhaust-gas-cleaning catalyst converters and particulate-matter-capturing ceramic honeycomb filters comprising ceramic honeycomb structures, which may be called “honeycomb structures.”
As shown in FIG. 2, a honeycomb structure 20 usually comprises an outer peripheral wall 21, and perpendicular cell walls 23 formed inside this outer peripheral wall 21 for defining a large number of cells 24. When used, such honeycomb structure 20 is placed in a metal container (not shown) with its outer peripheral wall 21 strongly gripped by holding members disposed on an inner surface of the container.
A cordierite honeycomb structure 20, for instance, is conventionally produced by the following steps. Cordierite-forming material powder, a molding aid, a pore former and water are first blended to form a moldable ceramic material, which is extruded from a die to provide a honeycomb molding integrally having an outer peripheral wall 21 and cell walls 23. This molding is placed in a drying furnace to evaporate water from the molding, and then placed in a sintering furnace to remove the molding aid, etc. It is then sintered to obtain a honeycomb structure 20 having fine pores in cell walls 23 with predetermined shape and strength.
For instance, when a large ceramic honeycomb filter for diesel engines, which has an outer diameter D of 150 mm or more and a length L of 150 mm or more in FIG. 2, is produced, there are such problems that when a honeycomb structure is formed from a moldable material by extrusion molding, its cell walls 23 near an outer peripheral wall 21 are deformed by its own weight, and that the honeycomb structure does not have sufficient strength after sintering.
To solve such problems, JP 2004-75524 A discloses a honeycomb structure having an outer peripheral wall obtained by removing a peripheral wall portion from a honeycomb body, and coating the resultant outer peripheral surface with a coating material comprising 100 parts by weight of ceramic powder comprising cordierite particles and/or ceramic fibers, and 3-35 parts by weight of an inorganic binder of colloidal silica or alumina. JP 2004-75524 A describes that this coating material provides the outer peripheral wall with improved peeling resistance, resulting in a honeycomb structure having excellent heat resistance and heat shock resistance. However, in the outer peripheral wall of the honeycomb structure described in JP 2004-75524 A, the migration of colloidal silica or alumina toward outer periphery is suppressed. As a result, the honeycomb structure has high strength not only on outer periphery but also inside, less absorbing heat shock applied to the cell walls, thus providing the outer peripheral wall with insufficient heat shock resistance.
JP 2006-255542 A discloses a honeycomb structure having an outer peripheral wall formed by a coating material comprising ceramic particles having an average particle size of 20-50 μm, which is provided with a dense surface layer as thick as 1-50 μm or an impregnated dense layer as thick as 10-300 μm by applying a coating agent comprising colloidal ceramics such as colloidal silica or alumina, etc. as main components to the outermost surface of the outer peripheral wall. JP 2006-255542 A describes that in a honeycomb structure having such dense surface layer or impregnated dense layer, the outer peripheral wall has smaller porosity on surface than in a center portion, resulting in less detachment of ceramic particles forming the outer peripheral wall, and excellent durability and wear resistance. However, because the outer peripheral wall of the honeycomb structure described in JP 2006-255542 A does not contain a binder, water in the coated material comprising ceramic particles having an average particle size of 20-50 μm is easily absorbed by the honeycomb structure, resulting in densification in the inner side of the outer peripheral wall. As a result, the outer peripheral wall has high strength on the inner side, so that heat shock applied to the cell walls is less relaxed, resulting in low heat shock resistance.
JP 5-269388 A discloses a method for producing a ceramic honeycomb structure by forming its outer peripheral wall with a coating material comprising 100 parts by weight of cordierite particles and/or ceramic fibers and 3-35 parts by weight of colloidal oxide as main components. It is described that this method can form an outer peripheral wall having heat shock resistance, strength and reliability on the ceramic honeycomb structure, while preventing the cracking of the outer peripheral wall that would occur in a drying step. However, because the outer peripheral wall of the honeycomb structure described in JP 5-269388 A is left to stand for 24 hours in the air for drying, the colloidal oxide does not move toward outside in the outer peripheral wall, so that the outer peripheral wall has high strength in both outside and inside. Accordingly, heat shock applied to cell walls is less relaxed, providing the outer peripheral wall with insufficient heat shock resistance.
Further, because large, heavy honeycomb structures having outer diameters of 150 mm or more and lengths of 150 mm or more for use in ceramic honeycomb filters for diesel engines are easily influenced by engine vibration and vibration due to contact with road surfaces, the outer peripheral walls described in JP 2004-75524 A, JP 2006-255542 A and JP 5-269388 A are insufficient in hardness, being likely to damaged during use. Particularly, ceramic honeycomb filters for diesel engines used in construction machines, etc. are subjected to very large vibration and shock during use, so that their outer peripheral walls are likely damaged. If the outer peripheral walls are made thicker to have higher hardness to prevent damage, they are easily cracked due to heat shock, resulting in low heat shock resistance.